The two workaround suggested (remove Avast and allow desktop running Avast to bypass proxy) don't work if you have a 'public' network where users bring their own laptops and connect to your network. In connecting, they can inadvertently bring down web access for all. Yikes.

Open ticket for just this scenario.

I found this suggestion:

In /etc/dansguardian-av/lists/bannedregexpheaderlist, add the following line:
User-Agent: avast.*
And then restart the ClearOS content filter:
service dansguardian-av restart